The present invention relates to a process for the preparation of chlorine dioxide, and more particularly, it relates to improved commercial processes for the economical preparation of chlorine dioxide.
Chlorine dioxide is quite useful industrially because of its very strong oxidizing properties. The principal areas of use are in the bleaching of paper pulps and of textiles, and in the treatment of water, for which chlorine dioxide permits preparing potable water having good organoleptic properties.
Different means have been employed for forming chlorine dioxide. In general, the formation of chlorine dioxide utilizes the reduction of an alkali metal chlorate, usually sodium chlorate, in an acid medium. The most usual reducing agents currently employed are sulfur dioxide, hydrogen peroxide and alkali metal chlorides.
In this last case, the basic reaction utilized in such processes can be represented by the following chemical equation: EQU 2ClO.sub.3.sup.- +2Cl.sup.- +4H.sup.+ .fwdarw.2ClO.sub.2 +Cl.sub.2 +2H.sub.2 O
In processes of this type, the acid introduced into the reaction medium is generally sulfuric acid.
The overall reaction thus brought about can be represented by the following equation: EQU 2NaClO.sub.3 +2NaCl+2H.sub.2 SO.sub.4 .fwdarw.2ClO.sub.2 +Cl.sub.2 +2Na.sub.2 SO.sub.4 +2H.sub.2 O
Some processes utilizing the foregoing reaction systems are described, for example, in French Pat. Nos. 1,443,755 and 2,003,223, or also by Canadian Pat. Nos. 993,158 and 976,726. It is necessary, according to these prior art processes, to operate in a very acid medium corresponding to acid concentrations, expressed in normality, of some 1 to 12 N. There accordingly arises the formation of an acid effluent comprised by a mixture of sulfuric acid and sodium sulfate in aqueous solution, for which it is very difficult to find a use and which is accordingly discarded after it has been neutralized.
However, it is possible to generate chlorine dioxide starting from an alkali metal chlorate, usually sodium chlorate, by introducing into the reaction medium hydrochloric acid. This acid acts to furnish at the same time the reducing agent and the source of acidity. The reaction brought about in this case where the alkali metal chlorate is sodium chlorate is as follows: EQU 2NaClO.sub.3 +4HCl.fwdarw.2ClO.sub.2 +Cl.sub.2 +2NaCl+2H.sub.2 O
However, when an amount of hydrochloric acid in excess of the stoichiometric quantity for the foregoing reaction is introduced, either locally or throughout the reaction medium, a parallel reaction occurs according to the following chemical equation: EQU NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O
In this reaction, there is no formation of chlorine dioxide, but rather only the formation of chlorine.
Some processes utilizing hydrochloric acid acting both to furnish the acid and as the reducing agent have been set forth in various patents. By way of example, such processes relying on the above-indicated reaction system have been described, particularly in French Pat. No. 1,015,040 and in Canadian Pat. Nos. 1,049,950; 956,783; and 956,784.
The disadvantages accruing to these processes arise in the need to use expensive apparatus. This is the case in particular with the process described in French Pat. No. 1,015,040, wherein the equipment for the generation of chlorine dioxide is comprised by six reactors placed in succession and wherein the material of construction must be titanium, a metal resistant to corrosion by the reaction mixture at its boiling temperature. Moreover, the total volume of these six reactors is large. Thus, for the production of chlorine dioxide on the order of one tonne an hour, the reaction volume is about 45 m.sup.3.
According to Canadian Pat. Nos. 1,049,950; 956,783; and 956,784, the production of chlorine dioxide can be realized in such apparatus functioning under sub-atmospheric pressures, and their strength must be accordingly increased as a consequence.